The present invention relates to a method of decoding coded image data from a data stream, particularly for the transmission of sequences of moving images, and for bringing the contents of a predicted-image memory up to date on the basis of the pixel values obtained from the decoding, wherein the image data are coded according to a coding method employing residual redundancy and the data stream includes unequivocally associatable starting code words for the frame and/or fields.
The digital transmission of sequences of moving images by pixel-by-pixel coding and transmitting the original images requires bit rates which cannot be approximately reached in an economical manner in networks involving a large number of users, such as, for example, the ISDN of the German Federal Postal Service. Therefore, efficient coding methods have been developed for television signals in order to transmit or store image sequences. These coding methods utilize redundancies from frame to frame to reduce the data rate and the data quantity required for the reconstruction of moving images. These methods become even more efficient if motion compensation is additionally employed. For example, at the meeting of the CCITT (International Telegraph and Telephone Consultative Committee) Study Group XV, Working Party XV/1, Specialists Group on Coding for Visual Telephony, held Nov. 7-10, 1989, in Tokyo, Japan, a standard for the transmission of images was issued as Document 584, bearing the CCITT Recommendation No. H. 261. The Decision by Study Group XV was expected in March of 1990.
In the coding method according to the cited CCITT recommendation, or in similar methods, the redundancy contained in the sequence or succession of original frames is reduced considerably in that essentially only changes in the original frame are being transmitted. A predicted-image memory is provided for the reconstruction of the sequence of moving images, with the content of the memory covering a complete frame, and the memory is brought up to date by means of pixel values reconstructed at the receiving end from the transmitted and decoded data.
The data reduction has the result that transmission errors lead to noticeable image interference in the decoder. The reason for this is that a bit error generally leads to a chain of improperly decoded pixels and, on the basis of frame-to-frame prediction, an error is visible not only for a brief moment but for a longer period of time.
To prevent this type error as much as possible, conventional data reduction methods for image sequences operate with error protection and error correction for individual bit errors or short error bundles. In order to reduce in the course of time an error that has already occurred, frame-to-frame prediction is also interrupted periodically in small image regions and the original content is transmitted in coded form.
Error correction systems fail, among others, if there are long error bundles and particularly in packet or ATM networks when a whole packet or one or more cells are lost. Although the periodic transmission of original information gradually reduces errors, it cannot prevent its annoying visibility.